1. Field of the Invention
This invention relates to optical systems, and more particularly, to a method and apparatus for demultiplexing optical signals having multiple wavelengths.
2. Description of the Related Art
Optical measurement and alignment systems, such as those used in lithography systems, use various types of optical devices. For example, lithography systems use multiple optical devices in performing the necessary alignments for semiconductor manufacturing. Optical alignments in a lithography system may be required to properly position a wafer, a mask, or both. Because of the small dimensions of optical wavelengths, such optical devices may provide the necessary alignment with a very high degree of precision. In addition to performing optical alignments, multiple wavelengths of light may be used to mitigate the effect of various thin-film layers (e.g., dielectric, resist, etc.) which may be present on a wafer and can cause a wavelength-dependent response. Observations of various process steps, various measurements, and the detection of surface defects through multiple layers during wafer inspection may also be accomplished using multiple wavelengths of light.
The various tasks discussed above may require the use of multiple optical wavelengths. However, these optical wavelengths may be combined into a single beam of light by an optical multiplexer. The beam of light may then be shined upon the wafer or mask to be aligned. A reflected or diffracted optical signal may then be received by another portion of the optical alignment device. Since both the original light beam and the reflected/diffracted light beam include multiple wavelengths of light, demultiplexing may be required. However, achieving sufficient isolation between the various wavelengths of the light beam may be difficult, and may in turn affect the accuracy of the particular task being performed. If a wafer or a mask cannot be accurately aligned to a certain tolerance, it may affect the dimensions of various devices that can be implemented on integrated circuit die that are to be formed on the wafer. Furthermore, if other tasks, such as process observation or defect detection cannot be reliably performed, it may affect the quality, efficiency, or cost of manufacturing integrated circuits. Thus, it is important for lithography systems that perform such tasks by combining multiple wavelengths of light onto a single beam that the wavelengths can be separated by a demultiplexer. However, optical demultiplexing systems can be both expensive and cumbersome.